The field of the invention is x-ray imaging systems and methods. More particularly, the invention relates to suppressing image artifacts that result from highly attenuating objects.
In a computed tomography system, an x-ray source projects a fan-shaped beam which is collimated to lie within an x-y plane of a Cartesian coordinate system, termed the “imaging plane.” The x-ray beam passes through the object being imaged, such as a medical patient, and impinges upon an array of radiation detectors. The intensity of the transmitted radiation is dependent upon the attenuation of the x-ray beam by the object and each detector produces a separate electrical signal that is a measurement of the beam attenuation. The attenuation measurements from all the detectors are acquired separately to produce the transmission profile at a particular view angle.
The source and detector array in a conventional CT system are rotated on a gantry within the imaging plane and around the object so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements from the detector array at a given angle is referred to as a “view”, and a “scan” of the object comprises a set of views acquired at different angular orientations during one revolution of the x-ray source and detector. In a 2D scan, data is processed to construct an image that corresponds to a two dimensional slice taken through the object. The prevailing method for reconstructing an image from 2D data is referred to in the art as the filtered backprojection technique. This process converts the attenuation measurements from a scan into integers called “CT numbers” or “Hounsfield units”, which are used to control the brightness of a corresponding pixel on a display. Cone beam CT systems are similar to so-called “third generation” 2D CT systems in that the x-ray beam fans out, or diverges, in the plane of the imaging slices. In addition, however, the x-ray beam fans out in the perpendicular direction to acquire attenuation data for a plurality of image slices. Cone beam systems are also characterized by a 2D detector array which acquires data simultaneously for each image slice.
Most of the commercially available CT systems employ image reconstruction methods based on the concepts of Radon space and the Radon transform. For the pencil beam case, the data is automatically acquired in Radon space. Therefore a Fourier transform can directly solve the image reconstruction problem by employing the well-known Fourier-slice theorem. Such an image reconstruction procedure is called filtered backprojection (FBP). The success of FBP reconstruction is due to the translational and rotational symmetry of the acquired projection data. In other words, in a parallel beam data acquisition, the projection data are invariant under a translation and/or a rotation about the object to be imaged. For the fan beam case, one can solve the image reconstruction problem in a similar fashion; however, to do this an additional “rebinning” step is required to transform the fan beam data into parallel beam data. The overwhelming acceptance of the concepts of Radon space and the Radon transform in the two dimensional case gives this approach to CT image reconstruction a paramount position in tomographic image reconstruction.
C-arm x-ray systems are employed when it is desired to image a stationary patient from many different angles. C-arm x-ray systems are commonly used in interventional procedures where x-ray images are required often during a medical procedure and the physician must have access to the patient.
In C-arm x-ray systems, the x-ray source and the x-ray detector array are arranged opposite each other on the ends of a semicircular, C-shaped carrier. The C-arm can be rotated in the direction of the C-arm circumference in an “orbital direction”, and in a direction perpendicular to this. The patient is positioned at the center of the C-arm and by rotating the C-arm about its two axes, x-ray views of the patient can be acquired at any desired angle. The C-arm x-ray system may be operated to acquire 2D fluoroscopic images from a desired view angle in real time as a medical procedure, such as catheterization, is performed. Alternatively, it may be operated to perform a volume CT scan or a tomosynthesis scan to produce a 3D image.
When a metallic object, such as a surgical clip, is present in a subject being imaged, artifacts will generally be present in the reconstructed image. Typically, these artifacts present as a series of dark and light streaks that emanate from metal objects in the subject and are hence commonly referred to as “streak artifacts”. These artifacts degrade the overall quality of the images produced and, as a result, can confound clinical diagnoses. A number of different physical processes lend themselves to the production of streak artifacts. First, the metal object results in beam hardening, which is caused by the polychromatic x-ray beam spectrum and its energy dependent attenuation coefficients. Additionally, the large attenuations coefficients associated with metallic objects results in a low photon count in the imaging system detectors. The resulting under-range detection in the imaging system hardware reduces the overall signal-to-noise ratio of the reconstructed images. Lastly, nonlinear changes in the measured data appear as low-frequency tail artifacts around the metal objects, as well as between the metal and other high density objects.
Previous methods for streak artifact correction include replacing the projection signal produced by the metal object with a synthesized projection based on neighboring projection samples. Such previous methods are described by, for example, D. Felsenberg, et al., in “Reduction of Metal Artifacts in Computed Tomography—Clinical Experience and Results.” Electromedia, 1988; (56):97-104. While these methods are effective in reducing the presence of streak artifacts, they also remove valuable information from the image. For example, information pertaining to the metal object itself is lost when using these methods. Furthermore, information about the region in immediate proximity of the metal object is also lost. There are many clinical applications where the interface between a metal object and the surrounding tissues is important, and thus the present methods of artifact suppression are inadequate.
Therefore, it would be desirable to have a system and method for controlling the impact of artifact-causing materials that does not result in significant data loss.